The present invention relates to an integrated optical technology; and, more particularly, to a thermo-optic tunable attenuator has a wide range of light attenuation region, a linear operating characteristic and a polarization independence characteristic for an input electrical signal, and the ability to operate at a speed of milliseconds.
In general, an optical attenuator serves to attenuate an intensity of an input optical signal and a tunable attenuator serves to control a degree that the optical signal is attenuated in an electrical and mechanical manner.
A conventional tunable attenuator includes an opto-mechanical optical attenuator, an optical attenuator using a micro electro mechanical system (MEMS) and a thermo-optic optical waveguide type of optical attenuator.
The opto-mechanical optical attenuator utilizes a technique which spaces two optical fibers at a small interval from one another and mechanically moves a variable absorption filter or a shielding film over the interval, to thereby absorb or shut off a portion of incident light, or a technique which collocates two side-polished optical fibers with each other and controls an interval therebetween, to thereby adjust an intensity of light to be transmitted through. Although, however, the opto-mechanical optical attenuator may have extremely good characteristics including an optical attenuation range higher than 50 dB, a reduced insertion loss and a linearity of 0.1 dB, it suffers from a drawback that it requires numerous bulky discrete components to decrease the operational speed on a second basis.
In addition to an excellent operation characteristic of the opto-mechanical optical attenuator, the MEMS based optical attenuator also has merits including an increased operation speed of microseconds and a reduced operation power. Unfortunately, the MEMS based optical attenuator suffers from a drawback that it is difficult to integrate with another optical waveguide device.
The thermo-optic optical waveguide type of optical attenuator is implemented through the use of operation characteristics of a thermo-optic modulator or thermo-optic switch, which may be used in an integrated optical circuit. The thermo-optic tunable attenuator using silica or polymer has been published, which can precisely control an optical attenuation degree in an operation speed of milliseconds, a polarization independence and powers less than several hundreds mW.
In practice, as the opto-mechanical optical attenuator and the MEMS based optical attenuator, an optical attenuator using an asymmetric Y-branch optical switch has a digital-like operational characteristic in which the optical attenuation degree is continuously decreased with an external drive signal to reach to a saturation state, extends the optical attenuation degree over 30 dB, and increases a degree of fault tolerance to facilitate fabrication processes. For this reason, studies of the optical attenuator using the asymmetric Y-branch optical switch are actively in progress.
In FIG. 1, a schematic pictorial view of a conventional tunable attenuator using a thermo-optic device is illustrated. As shown in FIG. 1, the conventional tunable attenuator includes an optical modulator having an asymmetric Y-branch waveguide 11 and a drive electrode 12 for applying a power, a main output port 13 for outputting an attenuated output from the optical modulator, a monitor port 14 for extracting a portion of the main output port 13 to monitor extracted results, and a dummy drain port 15, which is branched from the asymmetric Y-branch waveguide 11 in a curve shape, for removing a portion of input optical power. The conventional tunable attenuator has been published in the following paper: xe2x80x9cPolymeric Tunable Attenuator with an Optical Monitoring Tap for WDM Transmission Networkxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11, No.5, p590, May 1999, Sang-shin Lee, et.al.
As mentioned above, the conventional optical attenuator including the asymmetric Y-branch waveguide 11 and the drive electrode 12 attenuates an intensity of output light beam responsive to an applied power using a mode evolution scheme and feeds back a portion of output light beam to thereby stabilize the output light beam.
However, the conventional thermo-optic tunable attenuator suffers from a drawback that since a change in optical attenuation degree for an applied power is non-linear, it causes a considerable amount of power consumption at an initial change in the optical attenuation degree, thereby adversely affecting the energy efficiency of the tunable attenuator.
It is, therefore, a primary object of the present invention to provide a thermo-optic tunable attenuator which is capable of preventing a considerable amount of power consumption due to a non-linear change in optical attenuation degree for a power applied externally, to thereby achieve a high linearity, a wide range of optical attenuation region higher than 35 dB, an operation speed of milliseconds, a polarization independence and be configured for use in an integrated optical circuit.
In accordance with a preferred embodiment of the present invention, there is provided a thermo-optic tunable attenuator, which comprises a single mode channel optical waveguide; an optical waveguide of a Mach-Zehnder interferometer structure coupled with the single mode channel optical waveguide, the optical waveguide including two symmetric Y-branches; and two metal heating wires for applying power to one side of each of the two symmetric Y-branches, wherein the two metal heating wires are in a facing relationship with each other, wherein a refractive index of the optical waveguide is changed in response to a power applied to any one of the two metal heating wires.
Preferably, the thermo-optic tunable attenuator further comprises a first metal electrode for connecting the two metal heating wires in a series fashion; and a pair of second metal electrodes, each of which being connected to a corresponding one in the metal heating wires, for applying the power to the metal heating wires.